A long-term study to determine the essentiality/nonessentiality of histidine in the diet of college-age males is planned. Diets will contain a low amount of nitrogen and 10 to 15 mg of histidine per subject per day. About 90 percent of the dietary nitrogen will be supplied by crystalline L-forms of amino acids and diammonium citrate. Calories to maintain weight will be established during the pre-experimental period and remain constant thereafter. Parameters known or thought to reflect histidine status will be measured in blood and urine. Among these will be daily nitrgen balance, serum enzymes, indicators of anemia and blood cell formation, and plasma and urinary histidine and histidine-related metabolites.